After reading the effect of Witch Bolt from the D&D PH, it really seemed to me like an insanely powerful spell, especially when cast at 2nd level or higher.
I have had no experience with the spell, but had the initial intention to make it a prepared spell for my Eldritch Knight. Do its drawbacks outweigh its potential as compared to other 1st level damage spells?
It's a situational pick.
In perfect situations where your target is in vision and has no cover for its full duration, it's a good way of dealing a consistent 1d12 damage each turn. Note that using a higher level spell slot only increases the damage on the initial hit (the one that is determined with your attack roll).
There are probably better spells for the higher spell slot (or even the same level 1 slots), but at low levels it is a nice way to be conservative with your slots as it doesn't take a spell slot to continue doing damage.
Eventually when you get your Extra Attacks, witch bolt is going to lose its kick. Outside of these other conditions, witch bolt shines when something is vulnerable to lightning damage.
My advice: I love picking it up on my Eldritch Knight, but as my character progressed, I find it simpler to just whack things and cast spells when I need to get past an elemental weakness.
On subsequent rounds the spell only does 1d12 damage, independently from the spell slot used. After level 5, cantrips are better if you hit, after level 11, they are better all the time. They also don't require concentration.
Restraining spells usually require concentration too, so it is very hard to keep the target in range. The range itself is so low that most monsters could easily use their move to get to you, and beat you out of concentration or consciousness.
The initial damage is hardly better than Fire Bolt on level 1, and worse than Scorching Ray in a second level slot.
All these together mean Witch Bolt is quite weak. For levels 1-4, if you have a grappler in your party, it is ok, but not spectacular even then.
Later it is horribly underpowered.
For an Eldritch Knight:
Witch Bolt deals an average of 6.5 damage per round. A single longsword attack deals the same average damage with a Strength modifier of only +2.
Sure, the weapon attacks will miss X% of turns, but Witch Bolt will also miss X% of the time or less (ideally your weapon attack bonus is at least as high as your spell attack bonus.) What's more, Eldritch Knights gain spell slots very slowly (3 times as slow as spellcasters) so wasting a slot is a big problem.
Let's consider a 13 AC enemy (the expected AC for a CR 0 to 3 monster) and an Eldritch Knight with +3 Strength, +3 Intelligence wielding a longsword one-handed. With an attack bonus of +5, the Eldritch Knight's attacks will hit 65% of the time. Finally, let's assume you have 4 turns to deal damage.
Witch Bolt average damage over 4 turns: \begin{array}{l l} \text{Probability} & \text{Damage} \\ \hline 0.65 & 26 \\ 0.35 \times 0.65 & 19.5 \\ 0.35^2 \times 0.65 & 13 \\ 0.35^3 \times 0.65 & 6.5 \\ 0.35^4 & 0 \\ \end{array}
If you do the math, Witch Bolt has an expected damage of 22.55 over 4 rounds, and has an average cost of 1.5 spell slots. (Or put differently, about 1/3 of the time you have to pay 2 slots and lose 1 turn's worth of damage.)
On the other hand Magic Missile can't miss and deals an average of 10.5 damage, and three longsword attacks will add an average of 7.5 * 3 * 0.65 = 14.625 for a total of 25.125 over 4 rounds, and only costs 1 spell slot. So Magic Missile is already a better option even without taking critical hits into account. (Any of the weapon attacks can score a critical, but the following turns of Witch Bolt can't.)
Burning Hands would also do the same average damage as Magic Missile if the target fails its save, and you still get half damage if it succeeds. If you can hit 2 creatures, it's a no-brainer.
And these are very conservative numbers for a Fighter! Any Fighting Style other than Armor or Protection would increase the damage further. If we're talking about a variant human with Polearm Master or Crossbow Expert, Witch Bolt doesn't stand a chance.
If you use True Strike beforehand, the damage looks like this: \begin{array}{l l} \text{Probability} & \text{Damage} \\ \hline 0.8775 & 19.5 \\ 0.1225 * 0.8775 * 6.5 \\ \end{array}
There's only two outcomes since you're using 2 turns at a time. Anyways, the math works out to an expected 17.8 damage over 4 turns at an average cost of 1.14 slots. Less likely to be wasteful, but also less damage.
Against a very high AC enemy Witch Bolt might seem appealing since the damage on subsequent rounds is guaranteed, but your chance of missing and wasting the turn are also much higher. Enemies with high AC - especially at lower CRs - are usually wearing medium or heavy armor and don't have good Dexterity saving throws, so you're much better off relying on Magic Missile or Burning Hands.
In short, Witch Bolt is never a good option for an Eldritch Knight.
For a Wizard/Sorcerer: While they'll deal less damage with their cantrips than an Eldritch Knight does with weapon attacks, they get much better access to concentration spells (e.g. Sleep, Grease) and the spell slots to make use of them. At level 3, Flaming Sphere or Scorching Ray will kill things much faster, and Web becomes another candidate for concentration. At level 5 cantrips go up in power and you get very similar numbers to the Eldritch Knight's.
For a Warlock:
Witch Bolt is worse than Hex + Eldritch Blast in every practical situation. 1d10 + 1d6 add up to 9 damage per round on average. That's 23.4 damage over 4 rounds once you take the 65% hit rate into account, and costs 1 slot since Hex has no save.
On top of that Hex's ability to impose disadvantage on ability checks can help you or other party members hide by giving the enemy disadvantage on Perception checks, or help them shove or grapple that enemy by hurting their Athletics or Acrobatics.
The final nail in the coffin is that Witch Bolt is only good for one creature; Hex can easily be applied to 2 creatures or more, and in a dungeon can potentially last until your next short rest. The extra range on Hex and Eldritch Blast help make this possible since you'll be out of attack range more often and thus make make less concentration saves.
For a Warlock (Levels 1 - 5): Witch Bolt is still useless. (At 1st level Warlock only has 1 slot of 1st level, and then at 5th level, only having 2 slots of 3rd level to cast with. And if you're going for damage as a warlock Witch Bolt is even more useless.) In fact, for Wizard/Sorcerer (Levels 1-5) Witch Bolt is arguably even more useless, since for the early leveling for a Wizard/Sorcerer, you are solely using your Cantrips. Firebolt / Eldritch Blast with a Human Variant with Magic initiate feat + Hex > Witch Bolt. Witch Bolt on a 20th level Warlock is 5d12 worth of Lightning damage... or a mere 35 points of damage on average; compare that with Eldritch Blast with Agonizing Blast + Hex... on average (considering all four beams hit the target) you're looking at 44 points of damage. Bottom line: Eldritch Blast + Hex > Witch bolt in every situation where damage is your concern.
With Witch Bolt this is too costly of an option for any serious Warlock to even begin to consider utilizing anyways, combine that with its lackluster damage after the initial cast (1d12 per round which also cannibalizes your action. Instead of carrying on the initial 5d12 cast).
Bottom line (Wizard): As a wizard you have so many options at your disposal. For Wizard. Firebolt, Fireball, Hold Person/Monster > Witch Bolt... Hold Person/Monster is a much stronger option than wasting your concentration on Witch Bolt.
For Sorcerers however, (Note: This is assuming that you're either Storm Sorcery, Draconic Bloodline, or just about any Sorcerer archetype...). So, without any further ado. For Sorcerer. Proper use of Flexible Casting, Metamagic (i.e., Twinned Spell Disintegrate at 9th level opener. Which correct me if I'm wrong but I think that is a total of 38d6+40 Force damage... 192 points of damage on average). and from just that alone 9th level Witch Bolt's 63 points of damage on average drastically pales in comparison with the iconic max level Sorcerer opener. And that's not factoring in Flexible Casting, where you sacrifice all of your 4th level slots and lower to grant you more sorcery points, that you can use to make more slots of 5th level (a maximum of 8 slots of 5th level. while still having 16 sorcery points leftover). Basically everything in your sorcerer toolkit > Witch Bolt....
When you use witch bolt only the first attack requires an attack roll and spell slot. All damage on each turn after that only requires concentration. No attack roll and no spell slot.
So a level 1 warlock can deal damage over this 1 minute period from 10 lightning damage to 120 lightning damage with the cost of only 1 spell slot and 1 minute of concentration. The only thing not to do is miss your first attack roll. The rest is automatic hit.
